The present invention relates to the field of antenna systems, and, more particularly, to an antenna system having a spatial filtering surface for imparting a spatial filter taper transform.
Frequency selective surface (FSS) filters are commonly used with antenna systems for providing multiple frequency rejection bands. Some of these filters use dielectric substrates or other materials that are substantially transparent to electromagnetic signal transmissions. Some of the surfaces suggest elements that provide a number of frequency rejection bands. Other similar devices are formed as spatial filters that are positioned separate from an antenna or phased array antenna system. The filters are situated in the aperture plane for reducing the amplitudes of spatial sinusoidal field distribution in the main beam region of a radiation pattern associated with the antenna system. Some of the devices also include radiation absorbing material placed within the aperture plane or various elements within the aperture plane for modifying amplitude or filtering frequencies.
In commonly assigned U.S. Pat. Nos. 6,052,098 and 6,195,062, parasitic antenna elements are provided adjacent to an array of dipole elements of an antenna and formed as patterned conductor elements on one surface of a thin dielectric substrate. Feed elements for the driven dipole array comprise patterned conductor elements formed on an opposite surface of the substrate. The feed elements have a geometry with a mutually overlapping projection relationship with the conductors of the driven dipole elements to form a matched impedance transmission line to the dielectric substrate with the pattern dipole elements. Further addition of dipoles to that structure could provide a spatial filter surface for enhancing the reduction of sidelobes.
It would be more advantageous, however, if a spatial filtering surface can provide for magnitude and phase tapers and be applied to many different types of reflector antenna and phased antenna arrays made of elements with uniform weights where electronics required for the weights and amplitude and phase do not have to be implemented at the array level.
The present invention allows the application of an active spatial filtering surface for increasing antenna efficiency and controlling any amplitude taper and a reflection or transmission phase relative to a ground plane and antenna elements of the electromagnetic radiation at a selected frequency with respect to an angle of incidence to a dielectric substrate on which spaced, resonant dipole elements are positioned.
In accordance with one aspect of the present invention, the antenna system includes a ground plane and a plurality of antenna elements forming an antenna array. A spatial filtering surface is positioned adjacent the antenna array through which electromagnetic radiation to or from the antenna array passes. This spatial filtering surface includes a dielectric substrate and a plurality of spaced, resonant dipole elements positioned on the dielectric substrate. Each dipole element has dipole ends and an associated diode for controlling any amplitude taper and reflection and transmission phase relative to the ground plane and controlling electromagnetic radiation at a selected frequency with respect to an angle of incidence to the dielectric substrate.
In one aspect of the present invention, the dipole elements are arranged in a plurality of rows and equally spaced from each other within each row. An associated diode comprises a varactor diode having a capacitance for any associated dipole element. Bias lines interconnect the dipole elements for conducting a bias current to a dipole element and any associated diode. The bias lines can be formed of metal to aid in controlling voltage of any associated diode. The bias lines can also be formed as optical control lines.
In yet another aspect of the present invention, the antenna elements form a planar array and the spatial filtering surface is substantially parallel to the planar array. A dielectric filler is positioned between, above and below each dipole element positioned on the dielectric substrate.
In yet another aspect of the present invention, each dielectric layer comprises a plurality of dielectric layers. A dielectric layer can overlie the resonant elements. The dielectric layer overlying the resonant elements can be formed as a plurality of dielectric layers. The spatial filtering surface is also formed as a multilayer spatial filtering surface comprising a plurality of spaced dielectric substrates each forming a spatial filtering surface layer having the resonant dipole elements positioned thereon.
In yet another aspect of the present invention, a multilayer spatial filtering surface is set forth as a plurality of spaced dielectric substrates each forming a spatial filtering surface layer, with each layer including a dielectric substrate and a plurality of spaced, resonant dipole elements positioned on the dielectric substrate. Each dipole element has dipole ends and an associated diode for controlling amplitude taper and a reflection phase of an electromagnetic radiation at a selected frequency with respect to an angle of incidence to the dielectric substrate.